And those that overestimate the level of threat are doomed to paranoia.

Where are the attacks? There are 10's - no 100's - of thousands of sites such as bridges, rail lines, tunnels, places where people gather by the thousands, etc. etc. etc. Far too many for us to protect. No attacks.

I am far from certain that there are "many out there." We will be attacked again, I do believe, and successfully. One simply cannot make any nation - no matter how "free" or how much under the thumb of a police state free from fanatics intent on destruction.

As you point out, it was 19 guys with a plan and some box cutters who made a big strike. What is the exact evidence that there are more out there as smart or smarter?

IMO, we need to do our best without acting like cowards and old ladies. Then live free or die. (And my guess is that there will be hundreds of millions in the "live free" category and relatively few in the "die" category as time goes on.